field of the Invention PA1 Description of the Prior Art
The present invention relates to a digital frequency synthesizer receiver employing a phase locked loop. More specifically, the present invention relates to an improved digital frequency synthesizer receiver employing a phase locked loop structured to be controllable in a sequential manner for the purpose of scanning of a given receiving band.
A typical prior art radio receiver comprises as a local oscillator for a tuner a parallel resonance circuit comprising a coil and a capacitor, in which the inductance of the coil or the capacitance of the capacitor is varied to obtain a desired tuning frequency. However, such a local oscillator is liable to suffer from the fluctuation of the oscillation frequency caused by a temperature characteristic of the coil, capacitor and the other oscillator components. Thus, it is rather difficult to obtain a stabilized oscillation frequency.
A digital frequency synthesizer has also been proposed and in practical use. Such a frequency synthesizer is much more advantageous in that it can provide a much more stabilized oscillation frequency. A typical frequency synthesizer employs a phase locked loop, which is often simply referred to as "PLL".
A frequency snythesizer employing a phase locked loop usually comprises a voltage controlled oscillator the oscillation frequency of which is controllable as a function of an output voltage, as low pass filtered, obtainable from a phase detector, which is adapted to compare the phase or the frequency of the output from a reference oscillator and the phase or the frequency of the output from a programmable frequency divider adapted to frequency divide the output frequency from the said voltage controlled oscillator at the frequency division rate which is adapted to be variable as a function of the control signal. Automatic scanning of the oscillation frequency of the output from the said voltage controlled oscillator is effected by varying the said control signal and thus the frequency division rate of the programmable frequency divider. Therefore, if such a voltage controlled oscillator is used as a local oscillator of a tuner of a radio receiver, automatic scanning of the receiving band can be effected by varying the frequency division rate of the programmable frequency divider, as described above. If and when the frequency of a broadcasting station is tuned by the tuner, a reception output is obtained by the receiver, which is utilized to disable variation of the said control signal, thereby to establish a reception state of the receiver.
The variable range of the frequency division rate of the programmable frequency divider should be determined depending on the range of the local oscillation frequency of the local oscillator for a given receiving band, such as an AM medium wave band, AM short wave band, FM band, or the like and the frequency difference between the adjacent two broadcasting station frequencies, such that the receiver can receive any broadcasting frequency of the said given receiving band. Thus, it is appreciated that the variable range of the frequency division rate of the programmable frequency divider should be different depending on the receiving band. For example, the Japanese standard for FM broadcasting has been determined as the range of receiving frequencies being 76 through 90 MHz, the intermediate frequency being 10.7 MHz and an inter-station frequency being 100 KHz, which means that the local oscillation frequency should be varied 65.3 through 79.3 MHz. Accordingly, it is necessary that the frequency division rate of the programmable frequency divider is varied from 653 to 793. On the other hand, the Japanese standard for AM medium wave broadcasting has been determined as the range of receiving frequencies being 535 through 1605 KHz, the intermediate frequency being 455 KHz and the interstation frequency being 10 KHz, which means the local oscillation frequency should be varied from 990 to 2060 KHz and accordingly the frequency division rate of the programmable frequency divider need be varied from 99 to 206.
If a radio receiver employing a frequency synthesizer including a phase locked loop as discussed in the foregoing is adapted to be easy of precise setting of a receiving frequency and is provided with a display capable of displaying a digital representation of the receiving frequency, it would be more advantageous. If such a radio receiver is adapted to provide various performance such as display of the time and the like, it would be much more desirable. However, such provision in a radio receiver employing a frequency synthesizer is of a receiver concept totally different from that of the prior art and therefore this fact makes the circuit configuration and the operation thereof complicated and particularly makes difficult the control of the circuit for the purpose of the scanning operation of a receiving band. For example, it is extremely difficult to coordinate the controls in selection of automatic scanning and manual scanning, automatic start of scanning in response to band switching, presetting of tuning frequencies in the course of scanning operation, and the like in such a radio receiver employing a frequency synthesizer.